This invention relates to AC circuit breakers in general and more particularly to an improved method and apparatus for arc quenching in such circuit breakers.
Circuit breakers in which arc quenching is accomplished by having an arc chamber in which the arc rotates in a quenching medium between open electrodes, the ends of which are situated close together, are typically used in medium voltage and high voltage installations. In particular, they are used in installations where the distribution voltage is above 1000 V. As is well known in the art, medium and high voltage circuit breakers for AC current extinguish at the zero-crossing point of the current. While in compressed-gas breakers a physically confined switching arc is subjected to the flow of a quenching gas, in breakers with a rotating arc cooling of the arc is obtained by imparting to it fast motion in a stationary gas, preferably a quenching gas and particularly sulfur hexafluoride SF.sub.6. Breakers with a rotating arc require no pressure gradient in the quenching gas. Becuase of the rapid travel of the arc bases at opposite electrodes, in particular where open ring- or spiral-shaped electrodes are used, very little contact burnoff occurs and as a result, a long useful contact life is possible.
In one known embodiment of such a breaker disclosed in the journal "Elektrie," No. 10, pages 364 to 366 (1967), there is arranged in series with the switching gap a coil which produces a magnetic blasting field when the arc current flows through it and is thus referred to as a blasting coil. The contact system comprises two electrodes arranged concentrically to each other forming a ring gap in which the switching arc rotates in an arc-quenching medium under the action of the Lorentz force.
Shorted turns may also be associated with magnetic blasting coils. These cause a displacement of the magnetic field with respect to the current curve. As a result, the speed of rotation of the arc remains high as the current magnitude within a halfwave of the AC current decreases.
The arc is extinguished at the zero crossing of the current due to the cooling effect of the relative motion between the gas and the arc.
An arc-quenching arrangement which serves as an overvoltage protection device for capacitors is disclosed in German Pat. No. 1,228,334. The arc is initiated in a spark gap and rotates between ring electrodes arranged parallel to each other and whose beginning and end are situated next to each other in such a manner that they form a single layer, flat spiral. The electrodes which are situated opposite each other serve as running electrodes for the arc which rotates between them under the action of its own magnetic field until a mechanically moved insulating member used as a deflector for the arc, deflects and lengthens the arc to a quenching device consisting essentially of horn-shaped electrodes. The motion of the arc is increased between the horn-shaped electrodes by blasting coils, through which the current being interrupted flows.
The quenching of an arc by cooling at the surfaces of an insulator body is described in German Pat. No. 819,268. As disclosed therein, an inner electrode is coaxially enclosed by a ring electrode in AC breaker having a blasting coil whose inner turn forms the periphery of a cylindrical quenching chamber serving at the same time as the electrode for the arc. An arc is drawn between the electrodes using a movable contact and rotates between the electrodes on a circular path. Quenching of the arc by the cooling effect of the relative motion is further aided in this breaker by the cooling effect of the bottom and top of the flat quenching chamber.
Although these prior art methods and apparatus work reasonably well, there is a need for improved methods of arc quenching due to various deficiencies. Prior to summarizing the present invention, a discussion of the operation of gas flow breakers which operation is helpful in understanding the present invention will be given.
In what are referred to as gas flow breakers, i.e., high capacity circuit breakers using flowing gas as a quenching medium, in which the arc burns in nozzle arrangments, the switching capacity is heavily influenced by what is referred to as back-up effect. Back-up effect is a certain interaction between the quenching gas flow and the arc. In a gas flow breaker the arc burns between two contacts, at least one of which is generally shaped as a tubular contact and forms at the same time a nozzle for the gas flow. However, it is also possible for the contacts to be preceded by a separate nozzle. The basic characteristic of all these arrangements is that the arc must burn through a hollow space. The hollow space may be cylindrical, conical or Laval-tube shape and may be of different length. The quenching medium must flow through this hollow space forming the nozzle or acting as a nozzle. The quenching medium flow will be impeded by an arc therein. Using simple model concepts, two zones can be distinguished within the nozzle, an inner hot zone with low density and an outer cold zone with high density. The inner zone is formed by the arc while the major portion of the total mass passing through the nozzle flows through the outer cold zone. The thicker the arc becomes, the wider the hot zone becomes. This happens at the expense of the cold outer zone. As a result, with increasing arc thickness, the mass flow through the nozzle declines. If the arc completely fills the nozzle cross-section, mass flow is minimal.
Thus, flow resistance in the nozzle increases as a function of the power of the arc. At the same time, the mass flow through the nozzle decreases as a function of the temperature. The removal of the gas heated in the quenching chamber for the duration of the arc requires a certain amount of time, particularly where large currents are being switched. Furthermore, the gas flow adjusts itself only subsequently with a time delay because of the mass inertia. In an AC circuit breaker, the arc current varies in accordance with the sinusoidal waveform of the current half-waves. In interrupting a large current, particularly a short-circuit current, the effects mentioned above occur in the vicinity of the current maximum.
With increasing cooling-down of the arc, a radial gas flow inward occurs and thereby a corresponding increase in density. An additional annular cross section is thus made available for the gas flow. At the zero crossing of the current, the undisturbed steady-state flow which depends only on the pressure is not yet reestablished but is reduced by a predetermined amount. This flow reduction impedes the cooling of the arc and the development of a temperature distribution favorable for quenching at the important time interval just prior to the zero crossing.
The reduced cooling effect in dependence on the decreasing mass flow has a consequence that the power released in the quenching chamber cannot be removed by the quenching medium. One result of this is a large pressure increase in the quenching chamber. The pressure increase can lead not only to a reduction of the inflow from the high-pressure part of the circuit breaker, but even to a reversal of the flow direction. Through such a reverse flow, hot gas can get into the supply canals. If with a then decreasing magnitude of the current, the gas flow resumes in the desired direction, quenching medium which has already been heated and which may be contaminated with metal vapor from the electrodes flows first into the quenching arrangement. Because of these effects, designs which largely suppress the back-up effect using the special measures have been developed. Typical are the measured disclosed in ETZ-A, vol. 90, no. 26, pages 711 to 714 (1969). In gas flow breakers, the back-up effect is prevented primarily by making the discharge openings of the nozzles relatively large, in accordance with the power to be interrupted.